It was in 2004 that graphene first appeared in the laboratory, when scientists Andre Geim and Kostya Novoselov at the University of Manchester discovered that they could get a graphite flake which became thinner and thinner by a very simple method. They isolated a graphite flake from graphite, then stuck both sides of the flake to a special tape and peeled the tapes off, thereby cleaving the graphite flake into two parts. As they repeated this process several times, the flakes got thinner and thinner. Finally, they obtained a monolayer of carbon, which is known as graphene. From then on, numerous new methods have been explored to prepare graphene, among which the following two methods are most commonly used.
1. Chemical vapor deposition provides a controllable and effective method for preparing graphene. In a Chemical vapor deposition method, a flat substrate (such as a metal film, a monocrystal metal or the like) is exposed to the atmosphere of a precursor (such as methane and ethylene etc.) which is decomposable at high temperatures, then allowing carbon atoms to deposit on the surface of the substrate by annealing them at high temperatures so as to form graphene. Consequently, isolated graphene flakes are obtained by employing a chemical etching method to remove the metal substrate. The growth of graphene (such as growth rate, thickness and flake area etc.) can be controlled by selecting the parameters such as the type of the substrate, the growing temperature and the mass flow etc. However, the biggest disadvantage of this method is that the strong interaction between the graphene flakes and the substrate leads to a loss of many characteristics of the graphene monolayer, and weakens the continuity of the graphene.
2. Thermal decomposition of SiC provides another method for preparing graphene. In this method, monocrystal SiC is heated to make the SiC on the surface decompose so as to remove Si and form graphene by the residual carbon. However, the monocrystal SiC used in the SiC thermal decomposition is very expensive, and the graphene as grown takes on island-like morphologies and is porous, with the number of the graphene layers being non-uniform.
An example of the methods which are widely used now to prepare graphene is disclosed in Patent Literature 1: Firstly preparing a catalyst; then carrying out high-temperature chemical vapor deposition by loading a substrate into an oxygen-free reaction vessel, wherein the substrate carries a catalyst, heating the substrate to a temperature of 500 to 1200° C., and introducing a source of carbon-containing gas to begin the chemical deposition so as to generate graphene; followed by purifying the graphene to remove the catalyst by acid treatment or evaporation under low pressure and at high temperature. Nevertheless, this method has many drawbacks such as the complexity of the process, an additional removal of the catalyst being required, high energy consumption and high production cost.
Patent Literature 1: Chinese Patent Application No. CN101285175A.